The manufacture of printed circuit boards includes numerous sequential manufacturing steps in which the patterns of each successive manufacturing process must be congruent with the patterns produced by preceding processes. These successive manufacturing steps are normally all fairly complex in nature. They include pattern printing using a photographic mask, herein referred to as a phototool, and machining processes including drilling, routing, shearing, and sawing. The pattern and structure of the printed circuit board resulting from each step of these many sequential operations must be in accurate registration with patterns and structures generated in preceding process steps. This is particularly critical in multilayer boards where precise registration of the layers to each other is necessary to the proper electrical performance of the printed circuit board. A misregistration of a single operation could result in an electrical open or short circuit and nullify the correct work of all the other manufacturing steps.
Due to the trend to finer and finer grids and to closely spaced features, modern printed circuit boards require a very high degree of positional accuracy of imprinted features. Misregistration of pattern features on the panels may occur from misalignment of the panel and phototool or machine tool in any singular step of the manufacturing process. Another source of misregistration is deformation in the panel that may occur in one or more process steps or deformation in the phototools or machine tools used to process the panel. Temperature, humidity and repeated use alter the dimensions of the phototool, and wear and thermal stress may alter machine tool accuracy. In addition, the separate preprocessing of phototool and panels in different environments may introduce conflicting dimensional changes.
Due to the extreme importance of maintaining correct registration of all the features generated by the sequence of manufacturing steps throughout the entire sequence of the manufacturing operations, sampled product must be measured frequently during this process. Insufficient and infrequent inspection at various stages of manufacturing result in a very decreased yield of acceptable product. Overfrequent and excessive inspections may interrupt production unnecessarily and increase the cost of complex printed circuit boards beyond acceptable levels.
Conventional inspection techniques typically establish a sample size and measure the resulting features of the manufacturing process and identify those printed circuit boards that are acceptable and those which are not within tolerance within the lot sample. Frequent samples are necessary to identify quickly when the manufacturing process is out of control and limit the number of defective panels produced to a small number. Conventional frequent sampling is unfortunately costly and labour intensive. It typically does not indicate the aspect of the manufacturing sequence that has drifted out of control. Continued production of unacceptable product, therefore, is prevented by stopping the production process; but this method of statistical quality control is concerned with the quality of the printed circuit board and not the manufacturing process. It does not enhance or assist the identifying of the fault condition causing poor quality or directly promote rapid continuance of the production process.